In recent years, governments have become increasingly aware of the dangers to public health, resulting from exhaust fumes of internal combustion engines, particularly in confined spaces. Among the constituents of exhaust gases which give rise to concern are carbon monoxide, incompletely burnt hydrocarbons, nitrogen oxides and lead containing particulate matter. Regulations have been issued in many countries to reduce the amount of such emissions in exhaust gases. Attempts have also been made to satisfy the requirements of such regulations in several ways including:
(a) Internal combustion engines have been designed so that they operate in such a manner as to produce an inherently "cleaner" exhaust,
(b) Gasoline in which the added lead content is reduced or absent, is being sold on a wider scale than previously, and is mandatory in many areas,
(c) Attempts have been made to filter the exhaust, or to provide a catalyst which would lead to a more complete combustion of carbon monoxide and unburnt hydrocarbons.
While progress has been made with the first two of the methods set out above, they have the disadvantage that the development of a completely new engine for motor vehicles is extremely expensive. Thus, manufacturers would wish, as far as possible, to continue to produce engines whose development has already largely been carried out. In addition, the production of lead-free petrol means that refineries would need to be run in a less efficient manner. Further, said refineries use more crude oil than is possible when the octane rating of lower octane hydrocarbons can be increased by the inclusion of lead compounds.
In the instance of diesel engines, under normal operation they generate large amounts of particulate carbon. The latter are discharged into the atmosphere as a heavy, black exhaust stream.
At higher exhaust temperatures, the carbon is burned off. At lower temperatures, such as under idling conditions, the relatively cool temperature precludes such action.